Maybe your wi-fi router is broadcasting at higher channels (12, 13) and some countries regulations limit the channels to 1-11 only. You can do one of those:
- Change the channel on your wi-fi router (to lower).
- Install this: http://maemo.org/packages/view/iw-reg-set-jp/
and keep your country settings.
- Change your N900 country settings as you live in different country (for example Europe,Japan) so channels 12,13 will be enabled

I recommend the second choice. This way you can see all the routers no matter which country you are.